Conventionally, for a precise measurement of an angle of a target, (1) a voltage which becomes greater as the target in the radiation beam of a radar antenna shifts from a direction of the beam center to the peripheral section, namely, the angle error voltage has been measured and the magnitude of the measured value has been used.
As methods to generate such a voltage, there have used a monopulse method in which the sum pattern and the difference pattern are generated in the antenna, a conical scan method in which the radiation beam is successively rotated about the direction of the antenna axis, and a lobe switching method in which the radiation beam in the angle measuring plane is successively deflected with the center located at the direction of the antenna axis.
In this specification, we use the term "beam" to refer to a radio wave which, not only when emitted but also when received, has a particularly strong intensity in a specific direction.
Moreover, as other precise measurement methods, there have been (2) a method in which a plurality of antennas are located in the angle measuring plane with the distance therebetween each being slightly different from each other and the reception phase differences between the respective antennas are measured to obtain the angle and (3) a method, as described in the Japanese Patent Laid-Open No. 60-46477, in which two beams are formed with the oriented directions thereof being slightly shifted from each other in the vertical direction, the direction of the antenna axis at which these two beams intersect each other is set to be oriented to the middle point of the target and the image thereof, and the phase difference between the received waves (the composite wave of the direct wave and the indirect wave) associated with these two beams are measured, thereby the determining the height of the target with a low elevation angle.
However, in the methods of (1) and (2), in a case where the target with a low elevation angle exists over a relatively calm sea and the antenna beam includes the target and the image thereof generated by the surface of the sea, the interference between the direct wave from the target and the indirect wave(mirror reflection wave) passing the surface of the sea makes it impossible to correctly measure the angle. The third method involves disadvantages in that high accuracy is obtained only when reflection coefficient k of the surface of a sea is nearly equal to 1, and many ambiquities are obtained together with the true height of a target as the ratio of the antenna height to the wavelength increases.
To remove these problems, according to the present invention, there is provided a radar system having means wherein two beams are formed with the oriented directions thereof slightly shifted from each other in the vertical direction by use of an antenna, the direction of the antenna axis at which these two beams inersect each other is set to be oriented to the middle point of the target and the image thereof by use of a direction adjusting mechanism the phase differences between the received waves associated with these two beams are measured with respect to two frequencies, and the actual height of the target is determined from two sets of heights calculated based on the phase differences, each set including a plurality of heights, thereby measuring the height of the target with a low elevation angle (low elevation angle measurement).
In the present invention, the antenna axis is caused to aim at the mid point between a target and the image thereof. The resultant wave of a direct wave and an indirect wave is received through two beams, and then is processed through a predetermined procedure on the basis of the phase difference between these two beams. Therefore the radar system is still capable of eliminating influence of the indirect wave, for the reason which will be described later, even when the value of k is smaller than 1. Also, the aforementioned phase difference is measured with respect to two slightly different frequencies; and the true height of the target is determined by comparing two sets of series of data of target height calculated from these phase differences. Therefore the radar system is capable of eliminating ambiguity when the ratio of the antenna height to the wavelength is larger.
In addition, when the transmission is effected only with the upper beam of the two beams, there appears a difference between the transmission gains in the target direction and the image direction during the measurement of the angle of elevation, which leads to an advantage that the irradiation on the target (consequently, the interruption of the received signal) due to the null of the interference pattern is prevented. Moreover, the additional use of the monopulse angle measuring function leads to an advantage that the switch-over operation is available for the high angle of elevation. Furthermore, when the monopulse operation is conducted with the sum pattern for the transmission in the similar fashion as for the conventional method, there is provided an advantage that the transmission gain is improved and a vertically symmetrical curve is obtained for the angle error sensitivity curve.
In addition, the low elevation angle mode of the present invention can also be applied to an angle measurement of a high angle of elevation by use of the relationships between the received signal voltage ratio and the angle of elevation of the target due to two beams when the image does not exist in the beam. Moreover, as for the antenna (1), not only an antenna of the reflection mirror type but also an antenna of the pahsed array type may be adopted. Furthermore, two wavelengths .lambda..sub.1 and .lambda..sub.2 are used to remove the ambiguities; however, the ambiguities can also be removed by changing the height of the antenna in place of changing the wavelength. In addition, in an apparatus in which the monopulse angle measuring function is additionally included, the monopulse operation circuit may comprise both an elevation angle (EL) circuit and an azimuth (AZ) circuit.